I mean, if Malaysia elected a 90-yo Prime Minister, I guess we could have an 80-yo President? Biden would be 86 at the end of a hypothetical second term. That would be older than I any serving non-dictator/royal world leader I could find, even in history (surely there's someone older).
It's exceptional but not completely unheard of. Konrad Adenauer, Germany's first Chancellor after WWII, took office at the tender age of 73 and governed for more than 14 years. When he finally resigned in October 1963 (he was rather unwilling to), he was just two months shy of his 88th birthday. You also mentioned Malaysia's Mahathir Mohamad and there are most likely quite a few more (depending on who you count as "non-dictator/royal" of course).
To be honest, our rapidly aging societies should better come to terms with the idea that an octogenarian (or even a nonagenarian) can be involved in politics as well. Such a decision should be made on the basis of an individual's mental and physical constitution and not because he/she crossed a magical age threshold.
